1. Field of the Invention
The present invention relates to a method for producing a biaxially stretched film made of ethylene-vinyl alcohol copolymer (hereinafter, referred to as xe2x80x9cEVOHxe2x80x9d) having a small dry heat shrinkage coefficient, a small bowing coefficient and excellent shape stability. The present invention also relates to a biaxially stretched film obtained by this method and a laminate including this film.
2. Description of the Related Art
EVOH biaxially stretched films are widely used as a material having excellent gas barrier properties. A laminate that is obtained by laminating the EVOH biaxially stretched film and a thermoplastic resin film having excellent moisture resistance and mechanical properties, especially a polyolefin resin film, is used as a multilayered plastic packaging material in various fields. For example, such a laminate is widely used as a container having excellent oxygen barrier properties in the shape of bags, tubes, cups and pouches in various fields such as food, cosmetics, medical and chemical pharmaceuticals, and toiletry. In recent years, food containers, in particular, have come to be more often heated for sterilization before or after filling them with food. Therefore, a material that has not only excellent oxygen barrier properties, but also a small dry heat shrinkage coefficient is in demand.
The inventors of the present invention have found that when an EVOH biaxially stretched film having a small dry heat shrinkage coefficient is selected, and a polyolefin film as described above is laminated thereon, then the laminate is likely to be curled obliquely (hereinafter, this curl is referred to as xe2x80x9cS-shaped curlxe2x80x9d) with respect to the longitudinal direction (machine direction; MD). The in-depth examination of the inventors of the present invention made it evident that the larger bowing coefficient the EVOH film has, the more likely an S-shaped curl occurs. In the stretched film, considering the conventional knowledge that dry heat shrinkage and bowing are contradictory properties, it seemed difficult to control both properties at the same time.
The inventors of the present invention made in-depth research in view of these facts, and consequently found that when an EVOH having a specific composition is used and specific production conditions are selected, the dry heat shrinkage and the bowing of the EVOH biaxially stretched film can be suppressed at the same time. Thus, the present invention has been achieved.
A method for producing a biaxially stretched film of the present invention comprises: stretching a film made mainly of an ethylene-vinyl alcohol copolymer having an ethylene content of 15 to 70 mol % and a saponification degree of 80 mol % or more under heating; cooling the film; and subjecting the cooled film to a heat treatment; wherein a heating temperature during the stretching is 60 to 160xc2x0 C., a cooling temperature is a temperature 30 to 100xc2x0 C. lower than the heating temperature during the stretching, and a temperature for the heat treatment is 150 to 190xc2x0 C.
In a preferred embodiment, the stretching is simultaneous biaxial stretching.
In a preferred embodiment, preheating is performed at 60 to 160xc2x0 C. before the stretching.
In a preferred embodiment, a stretching ratio in a longitudinal direction is 2.5 to 4.5, a stretching ratio in a transverse direction is 2.5 to 4.5, and an area stretching ratio is 7 to 15 in the biaxially stretched film.
In a preferred embodiment, a bowing coefficient of the biaxially stretched film is 15% or less.
An ethylene-vinyl alcohol copolymer biaxially stretched film of the present invention is produced by the above-mentioned method, and the film has a dry heat shrinkage coefficient in a longitudinal direction of 4.0% or less.
A laminate of the present invention comprises a layer made of the ethylene-vinyl alcohol copolymer biaxially stretched film and a layer made of another thermoplastic resin.
In a preferred embodiment, the thermoplastic resin is a polyolefin.
Thus, the present invention described herein makes possible the advantages of, providing a method for producing a biaxially stretched EVOH film having a small dry heat shrinkage coefficient, a small bowing coefficient, and excellent properties such as shape stability in a stable manner and in mass quantity; providing a biaxially stretched film obtained by the method having a high utility for packaging; and providing a laminate including a layer made of the biaxially stretched film and a layer made of another thermoplastic resin that is in contact with the above layer, and suitably used as a packaging material.